Currently, the only known risk factor for HIV-associated cryptococcal disease (cryptococcosis, CD) is profound loss of CD4T cells, but this cannot discriminate HIV-infected (HIV+) patients who will develop CD from those who will not. There are no biomarkers for CD in HIV-uninfected patients. Our group discovered that HIV+ individuals with a history of or who later developed CD had lower levels of IgM memory B cells than those who never had CD and that a reduced level was a strong independent predictor of CD status. IgM memory B cells, known to be depleted in HIV, produce natural IgM (nIgM) that binds conserved microbial determinants and provides ready-made pathogen defense. Thus, IgM memory B cells could protect against Cryptococcus neoformans (CN). Support for this concept comes from studies from our laboratory demonstrating that mice which lack serum IgM (secretory, sIgM-/- mice) exhibited reduced survival after pulmonary infection with CN than IgM sufficient mice, which was associated with reduced alveolar macrophage phagocytosis of CN that increased with adoptive transfer of nave serum IgM. Although much is known about acquired antibody (i.e. from passive or active immunization) protection against CN, the role of B cells in natural resistance to CD is an enigma. This application proposes to determine whether nIgM and the B cells from which it is derived mediate protection against CN. We propose studies in mice to determine the role of mouse homologs of IgM memory B cells, B-1 B cells, and their product nIgM, in immunity to CN, the mechanisms that govern their activity, and human studies to determine whether IgM memory B cell expression is a suitable biomarker for CD and seek CD-associated genes. The following aims are proposed: 1) To determine the role of B-1 B cells in protection against CN in mice; 2) To identify mechanisms by which B-1 B cells and/or nIgM potentiate immunity to CN; 3) To link IgM memory B cell expression to human CD and seek CN-associated molecular profiles. These aims will have an impact on clinical medicine, informing biomarker discovery to overcome barriers to early diagnosis, development of new vaccines and therapies to overcome barriers to effective prevention and treatment, and impact basic science by revealing novel mechanisms of CN-host interaction. PUBLIC HEALTH RELEVANCE: Cryptococcosis, which is caused by the fungal pathogen, Cryptococcus neoformans (and other Cryptococcus species), is a life-threatening complication of HIV infection, causing > 900,000 cases and > 600,000 deaths annually globally, principally in sub-Saharan Africa and South-east Asia. The devastation of HIV-associated cryptococcosis is even greater because it is nearly impossible to cure in people with HIV infection. Cryptocococcosis also occurs in patients with solid organ transplants and in seemingly normal people. At present, there is no way to identify patients with HIV/AIDS who will get cryptococcosi and there are no known risk factors in people without HIV/AIDS or who have transplants. A study from our laboratory uncovered a previously unsuspected association between the loss of a certain subset of B cells, IgM memory B cells, and HIV-associated cryptococcosis. IgM memory B cells are depleted in HIV infection. The goal of this application is to use mouse models to identify mechanisms by which (the mouse homolog of) IgM memory B cells might potentiate protection against Cryptococcus and to perform human studies to determine if loss of IgM memory B cells is a risk factor for cryptococcosis. The public health significance of this work is high. It has the potential to identify biomarkers of cryptococcal disease and inform the development of new vaccines and therapies, which are greatly needed, especially in the developing world where the burden of disease is staggering. .